terminalwriter: remove TerminalWriter's stringio argument

Had a mark indicating it should be removed, and I agree, it's better to
just use the `file` argument.

testing/code/test_excinfo.py

@@ -1,3 +1,4 @@
+import io
 import operator
 import os
 import queue
@@ -1037,10 +1038,11 @@ raise ValueError()
         """
         )
         excinfo = pytest.raises(ValueError, mod.f)
-        tw = TerminalWriter(stringio=True)
+        file = io.StringIO()
+        tw = TerminalWriter(file=file)
         repr = excinfo.getrepr(**reproptions)
         repr.toterminal(tw)
-        assert tw.stringio.getvalue()
+        assert file.getvalue()
 
     def test_traceback_repr_style(self, importasmod, tw_mock):
         mod = importasmod(
@@ -1255,11 +1257,12 @@ raise ValueError()
         getattr(excinfo.value, attr).__traceback__ = None
 
         r = excinfo.getrepr()
-        tw = TerminalWriter(stringio=True)
+        file = io.StringIO()
+        tw = TerminalWriter(file=file)
         tw.hasmarkup = False
         r.toterminal(tw)
 
-        matcher = LineMatcher(tw.stringio.getvalue().splitlines())
+        matcher = LineMatcher(file.getvalue().splitlines())
         matcher.fnmatch_lines(
             [
                 "ValueError: invalid value",